1. Field of the Invention
The invention pertains to a method for optical tests, and to a system of tests to use the method, designed especially for testing the polishing of large-diameter precision mirrors, namely precision mirrors with diameters of more than 1 meter.
2. Brief Description of the Prior Art
Precision mirrors are used, for example, in telescopes or more generally, in optical systems designed for measurement and/or observation. Mirrors of this type have to be flawless, and their polishing has to be subjected to several tests during manufacture, these tests being generally done by interferometry.
Interferometry uses a light source, placed at the focus of the mirror, which may sometimes be at about 30 meters from the mirror. Once the distance becomes great, the tests may be disturbed by a variation in the refractive index of the air caused by air movements such as air currents, convection movements or heat stratification phenomena. Once the diameter of the mirror exceeds two meters, it becomes extremely difficult to make the measurements, and there are no known answers to this problem when the diameter exceeds four meters. Furthermore, methods of the prior art which can be applied to small-diameter mirrors are difficult to apply or, when it is still possible to use them, they prove to be costly.
Thus, there is the prior art method of measurement under partial vacuum in which the entire equipment has to be enclosed in a vacuum chamber. This system is heavy, bulky and costly. In practice it is difficult this method to apply to mirrors with diameters of more than three meters, because of the power needed for the vacuum pumps and the extra size that must be given to the walls of the chamber so that thes can bear the external pressure.
In a second method, the tests are conducted in a helium atmosphere: helium has a refractive index that varies very little with the temperature, and it possesses better light-propagating characteristics. However this method implies removing the air when filling the chamber with helium. This fact creates problems similar to those of the method of measurement under partial vacuum, and the helium must be recovered after measurement so as to re-use it for the next measurement for, if an aberration is detected in the mirror during a measurement, the required rectification must be made immediately. Thus, there may be several tens of measurements to be made when manufacturing a mirror, constantly raising the problem of removing air, filling the enclosure with testing fluid and recovering it.
An object of the invention, therefore, is a method which can be applied to all types of mirrors, especially large-diameter mirrors, enabling measurements to be made in a helium atmosphere or, more generally, in a fluid in which the path of the light rays is not disturbed, a method which requires a low-weight and compact system for its application.
3. Summary of the Invention
The invention pertains to a method to test the polishing of a mirror by means of a testing device in a fluid through which light is propagated homogeneously, a method wherein the mirror is placed on a support, this support is fixed imperviously to one end of a chamber which is flexibly shaped (i.e. capable of changing its shape) and/or folding in such a way that its internal volume can be eliminated, the testing device being at an opposite end of the chamber, a method wherein the residual air contained in the chamber is driven out by eliminating its internal volume, then a volume of fluid is injected into the chamber so that the polishing can be tested under efficient optical conditions, the said testing being done before the support and the mirror are removed in order to correct faults or to perform any other operation, for example its final packaging.
The invention also pertains to a system for the use of the method, comprising a support for the mirror, a testing device and a flexibly shaped and/or folding chamber, at one first end of which is fixed the testing device, means for the impervious fixing of the support to an opposite end of the chamber, means to remove the residual air from the chamber, means to inject the fluid into the chamber and means to remove the fluid from the chamber.
According to another characteristic of the method of the invention, the operation to inject fluid and remove residual air is done by leaving the measuring device and support in the position that they occupy for the measurement.
The method of the invention is, therefore, especially advantageous since, to remove any residual air in the chamber, it is enough to perform an operation comparable to the deflation of the chamber (an operation facilitated by the external atmospheric pressure), this operation being done while leaving the measuring device and support in the position occupied by them for the measurement. According to another characteristic of the invention, in order to quickly drive the air out of the chamber, a depression is created within it using a vacuum pump.
According to another characteristic of the invention, after making a measurement, the fluid used for the test is recovered before the mirror is removed from the chamber for correction or for any other operation.
In a system to use the method of the invention, the walls of the chamber at least partly comprise panels made of a highly resistant cloth impervious to the fluid, and their shape is such that they can take a position in which there is no remaining air pocket and a position in which a measuring device can be used to test the entire surface of the mirror.